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HAS the oil industry been exploring in the wrong places? Geochemists in
California are claiming that our ideas about how oil forms are mistaken, meaning
all bets are off about where to look for new deposits.

Oil forms from source rocks called black shale, which are made from organic
matter that settled on the ocean floor and was then compressed. Once organic
matter decays, it dissolves into the seawater and doesn鈥檛 accumulate. So
researchers always assumed that black shale would be found in places where life
once thrived in the ocean, but where a lack of oxygen near the seabed stopped
chunks of dead material from decaying.

But Martin Kennedy from the University of California, Riverside, has other
ideas. He believes black shale is made from fragments of clay minerals that
trapped dissolved organic matter, then sank and concentrated it on the ocean
floor. If he鈥檚 right, oil explorers should be mapping out where these clay
minerals were likely to be millions of years ago, and hunting there for ancient
deposits of black shale that have been compressed and heated to form oil.

Kennedy and his team focused on a kind of clay mineral called smectite. It鈥檚
made of layers of silicates and acts like a sponge鈥攕ucking up water and
tiny dissolved organic molecules into the gaps between its layers. If smectite
was taking organic matter down to the seabed, the researchers realised, then oil
is made from organic matter that has already decayed. This means that the oceans
didn鈥檛 have to be barren of oxygen鈥攕omething oceanographers have had a
hard time explaining. It would also explain why most of the carbon in the black
shale looks like amorphous gunk rather than identifiable pieces of dead organic
matter.

The researchers looked at the world鈥檚 most studied black
shales鈥攐utcrops in Wyoming and South Dakota that were underwater millions
of years ago. They found that the amount of carbon in a given slice was directly
linked to the total surface area of the smectite, suggesting that the clay had
been involved in trapping the carbon.

Tom Pederson, a geochemist from the University of British Columbia,
Vancouver, says he鈥檚 persuaded by Kennedy鈥檚 conclusion that clay is an important
factor. But he adds that the amount of organic matter being produced by life in
the ocean must still play a big part in determining where oil forms. 鈥淭he carbon
still needs to come from somewhere.鈥�

Using the results to find oil won鈥檛 be easy, admits Kennedy, since mapping
the clay would be incredibly complicated. 鈥淪mectite coming down the Amazon today
goes as far as Trinidad,鈥� he points out. And just because source rock has formed
in a given location doesn鈥檛 guarantee that it has turned to oil or that the oil
was trapped.

But oil companies are likely to take any leads very seriously. Kennedy began
his work at ExxonMobil, which still has a research group working to exploit the
idea. Now that Kennedy has published his work, other companies will be able to
take advantage of it too. 鈥淭he presence of source rock is fundamental to oil
exploration,鈥� says William House, a geologist with BP in Aberdeen. 鈥淧eople are
going to be very interested in reading this.鈥�